Typical cable television system networks, such as for cable TV or CATV, are normally comprised of four main elements, a head end, a trunk system, a distribution system consisting of coaxial feeder cables bridged from the trunk system, and subscriber drops fed from broadband signal taps coupled to the feeder cables in the distribution system.
The head end, the central originating point of all signals carried on the network, receives disparate signals from satellite, over the air broadcasts, or cable transmissions and combines and transmits these signals as a broadband signal (generally within a range of between 5 MHz and 1 Ghz) to numerous distribution nodes via trunk cables. Each distribution node, in turn, distributes the broadband signal along with power via the coaxial feeder cables, each feeder cable terminating at a termination block. Each feeder cable has numerous broadband signal taps coupled between the distribution node and the termination block. Each of these broadband signal taps draws a portion of the broadband signal and power (hereinafter, reference to signal means both signal, and the power supplied through the same cable) for use by a subscriber, typically a home or business, and transmits the remaining broadband signal to the next tap in the feeder. The portion of the broadband signal drawn (or tapped) from the main signal is transmitted via one or more drops to a like number of subscribers. Typically, there are hundreds, and sometimes thousands, of end users or subscribers connected to the coaxial feeder cables from one node.
In a subscriber's home, the drop can terminate directly into the television receiver on 12-channel systems, or into a converter where more than 12 channels are provided. Many newer cable-ready receivers and videocassette recorders have such a converter integrated into the tuner, with a switch to enable the additional channel tuning capability. Systems providing pay services may require a descrambler in the subscriber's home to allow the viewer to receive the special channels. Some systems use addressable converters or descramblers, giving the cable operator control over the channels received by subscribers with a computer. Such control enables impulse viewing or pay-per-view television service without a technician visiting the home to install the special service.
While the main purpose of cable television is to deliver a greater variety of high-quality television signals to subscribers, there is a growing interest in interactive communications, which allow subscribers to interact with the program source and to request various types of information, such as video text. An interactive system also can provide monitoring capability for special services such as home security. Additional equipment is required in the subscriber's home for such services. Monitoring requires a home terminal, for example, whereas information retrieval requires a video text decoder or modem for data transmission.
The broadband signal taps are coupled in series along each feeder cable, where the downstream side of a section of feeder cable is connected to an input terminal of the broadband signal tap and an output terminal of the tap is connected to the upstream side of the next section of feeder cable and so on. Within the broadband signal tap the broadband signal is split by a broadband signal transformer into a main signal and a much smaller tapped portion. The small portion of the broadband signal is delivered to the subscriber through a subscriber drop cable port, termed an "F-port", which connects to his drop. The remainder of the main broadband signal is returned to the coaxial feeder cable via the tap.
A conventional broadband signal tap comprises two assemblies: a housing and a cover or faceplate. The housing is for environmental protection of the tap electronics and further provides mechanical and electrical connection to the feeder cable. The tap housing attaches to the feeder cable with an input terminal and an output terminal. These terminals typically include electrical connectors for engagement with corresponding electrical connectors on the cover so that, when the cover is installed to the housing, the broadband signal and power delivered by the feeder cable is then routed through the cover. The faceplate contains electronics that apportions the broadband signal, sending a small portion of the signal to the subscriber and returning the main portion to the feeder cable through the output terminal.
An advantage of prior art broadband signal taps is that they allow for the removal and reinstallation of the tap cover and its corresponding electronics, without the need to remove and reinstall the entire broadband signal tap; that is, without the need to mechanically disconnect the tap housing from the feeder cable. These prior art broadband taps allow for the changing of the covers to provide for a different number of F-ports for more subscribers per tap, to upgrade the tap electronics and to provide replacement electronics for taps which become nonoperational.
However, networks such as described above having the broadband signal taps coupled in series along the feeder cables have some shortcomings. Namely, if one of the broadband signal taps goes out of service, e.g., by having its cover removed, the subscribers downstream of that broadband signal tap can be deprived of service. Such an interruption can occur as a result of lightning, mechanical or electrical failure, or even if a service technician removes the cover from its housing in one of the broadband signal taps. Removing the cover breaks the signal and power electrical continuity between the housing input terminal and its output terminal. This problem is aggravated if telephone service is provided over the same cable network. Interruptions to telephone service being much less tolerable than interruptions to television service.
More than just an inconvenience, service interruptions, even if for just a few milliseconds, can have serious consequences for the transmission of digital data over cable systems. Unlike analog signals, the loss of even a short portion of a digital data transmission can render the transmission unintelligible, or cause a failure of the signals to reach their intended destination. This probability increases directly with the bit rate and for high speed data transmissions even a short interruption is not acceptable.
Moreover, many of the newer services, including data and telephony services are being delivered by network interface units (NIUs) which are powered from the subscriber drop and thus from the tap. These NIUs include power supplies which provide for small variations in the power supplied over the drop but can not regulate for the loss of power caused by the removal of a cover of one of the taps in its network connection. A loss of power to a NIU could result in the loss of data downloaded to the device or other service interruptions.
Therefore, with the advent of pay-per view and interactive television, and the increased use of cable systems for telephone voice and data transmission, it has become much more important to provide a broadband signal tap which maintains continuity of service to the subscribers. In addressing this problem, it should also be borne in mind the substantial existing infrastructure of broadband signal taps which are currently in use along feeder cables.
Accordingly, there is an increasing need for a broadband signal tap with a means for maintaining the continuity of the broadband signal and the power over the feeder cable when the tap cover is removed from its housing.
It would be additionally advantageous to provide such a means for maintaining the continuity of the broadband signal and the power when the tap cover is removed from its housing which is easily retrofitable into existing broadband signal taps.